1. Field of the Invention
The present invention relates to a process for producing a sheet for electrophoretic display, a sheet for electrophoretic display and its use.
2. Description of the Related Art
An electrophoretic display device displays various data such as character data and image data by behavior of electrified fine particles when a voltage is applied, the electrified fine particles being, for example, dispersed in a dispersing medium. If the electrified fine particles and the dispersing medium are colored in different colors, when the electrified fine particles migrate to the surface of the dispersing medium by voltage application, a color of the electrified fine particles is observed and, when the electrified fine particles migrate to the bottom of the dispersing medium, a color of the dispersing medium is observed. If many electrodes which can apply a voltage by addressing are provided, different colors can be displayed every address, and any character data or image data can be displayed. The display data can be renewed and, at the same time, if an electric signal has not disappeared, the display data can be retained as they are.
As the electrophoretic display devices, in addition to stationary devices such as ordinary displays, some techniques of flexible displays have been proposed, such as electronic papers and electronic books, which have the shape of a thin sheet, which are ultraportable, and which can freely be bended.
For such electrophoretic display devices, there has been proposed a technique of microcapsulating a dispersion of electrified fine particles, that is, a dispersion for electrophoretic display. A dispersion for electrophoretic display is encapsulated in fine spherical microcapsules composed of a transparent resin or the like, and the resulting microcapsules for electrophoretic display are supported on the surface of a substrate sheet. Thus, a sheet-shaped electrophoretic display device having flexibility can easily be obtained. The dispersion for electrophoretic display is not locally distributed, or does not move, and this becomes a technique suitable for utility in which posture at use is variously changed, or display devices are bended, such as electronic papers.
In order to prepare a sheet for electrophoretic display utilizing such a microcapsule technique, various coating techniques are employed. As a means for coating microcapsules for electrophoretic display, there have been known, for example, a die coater and an air knife coater.
As a technique of coating various coating solutions with high productivity on an industrial scale, there has previously been known the roll coater technique of holding a substrate sheet between a pair of rolls to run the sheet, and transferring a coating solution, which has been attached to one side roll, to the substrate sheet. There has also been known a gravure coater using a gravure roll in which a gravure pattern consisting of fine irregularities is formed on a coating roll. The gravure coater has also been utilized in coating or printing on a pattern.
The microcapsules for electrophoretic display are desired to be supported substantially in a monolayer form and substantially densely on a substrate sheet. If two or more microcapsules are overlaid with each other, or if there is a great gap between microcapsules, unevenness in display properties or display pixel missing may occur, and this may greatly reduce display quality.
Further, as a sheet for electrophoretic display, microcapsules are supported on the entire surface of a substrate sheet, or alternatively, microcapsules are not present on a part of the surface, and the surface of the substrate sheet, or an electrode or an electrically conductive layer formed on the surface of the substrate sheet are exposed. Examples thereof may include the case where one wants to display character data or image data only on a part of the surface of a substrate sheet, the case where one wants to utilize a part of a substrate sheet for a purpose other than display, and the case where one wants to provide an electrode terminal for connecting wiring for applying a voltage to a sheet for electrophoretic display. In this manner, in order to support microcapsules only on a part of the surface of a substrate sheet in a prescribed pattern shape, some device is necessary.
For example, Japanese Patent Application Publication No. 2002-526812 discloses a technique of supporting microcapsules for electrophoretic display on the surface of a substrate sheet by a slot die coating method or an air knife coating method. In addition, Japanese Patent Application Publication No. 2002-365668 discloses a technique of supporting two or more kinds of microcapsules for electrophoretic display on a prescribed pattern position on a substrate utilizing photolithography technique. In this technique, a photoresist material is blended in a microcapsule dispersion, and a layer of the microcapsule dispersion coated is pattern-formed on a substrate.
However, by the above producing technique, it was difficult to prepare, with high productivity, a high quality sheet for electrophoretic display in which microcapsules for electrophoretic display are arranged in a prescribed pattern shape. In particular, in a type of usage such as electronic papers, it is required that a stable high quality sheet for electrophoretic display is produced on a large scale from an economical viewpoint, but there has been found no technique which is suitable for such an industrial production.
For example, in a gravure coater, it is difficult to coat a thick film. When a gravure roll having great irregularities of a gravure pattern is used, a trace of a gravure pattern remains on a microcapsule coated film which has been coated on a substrate sheet, and thus, it is difficult to coat microcapsules uniformly. In addition, a gravure roll is strongly brought into contact with a substrate sheet, so that an electrically conductive layer formed on a substrate sheet may easily be damaged.
In an air knife coating method employed in the technique of Japanese Patent Application Publication No. 2002-526812, microcapsules cannot be supported only on a part of a sheet surface. In a slot die coating method, although microcapsules can be supported in a pattern manner by devising a slot shape, in order to discharge a microcapsule coating solution through a narrow slot die, it is necessary to use a pump having a high discharge precision, and microcapsules passing through such a pump having a high discharge precision may easily be damaged. If microcapsules undergo some damage, they do not function as an electrophoretic display device.
The technique of Japanese Patent Application publication No. 2002-365668 utilizes a photolithography technique in order to arrange microcapsules for electrophoretic display in a pattern manner, and therefore, it is suitable for fine pattern formation. However, it requires complicated apparatuses or facilities and treating steps, so that producing efficiency becomes low and the cost of production becomes increased.
Under these circumstances, it is an object of the present invention to provide a process for producing a sheet for electrophoretic display which can support, with high efficiency, microcapsules for electrophoretic display only on a part of the surface of a substrate sheet in a precise pattern shape, a sheet for electrophoretic display obtained by the process, and its use.
In addition to the above problems, in the pervious producing technique, there is also a problem that it is difficult to handle the resulting sheet for electrophoretic display.
As described above, if a microcapsule coating solution is coated, and sheets for electrophoretic display immediately after drying are, as they are, wound in a roll manner, or overlaid, a microcapsule layer is brought into contact with the surface of a substrate sheet of an adjacent sheet for electrophoretic display, and microcapsules may easily cause blocking against the substrate sheet. When one tries to pull out a sheet for electrophoretic display from a wound roll in order to use it, microcapsules attached to the back of a substrate sheet may be damaged or detached.
Thus, it was contemplated that, when a sheet for electrophoretic display is wound in a roll manner, a releasing agent is coated on the surface of a microcapsule layer. In addition, it was contemplated that a sheet for electrophoretic display is wound by putting a sheet with a releasing agent coated thereon between the layers of the sheet for electrophoretic display. It was contemplated that this can avoid blocking between the microcapsules and the back of a substrate sheet and, when a sheet for electrophoretic display is pulled out from a wound roll, the microcapsules may easily be separated from the back of a substrate sheet, and the microcapsules cannot be damaged.
However, a problem arises that when one tries to use a sheet for electrophoretic display by attaching another member to the surface of the sheet on which side a microcapsule layer is formed, while a releasing agent is attached on a microcapsule surface, sufficient adhesiveness or attachment strength cannot be obtained due to the action of a releasing agent.
Therefore, it is another object of the present invention to provide a sheet for electrophoretic display having excellent handling properties such that, when the resulting sheet for electrophoretic display is used by pulling out from a wound roll, microcapsules cannot be damaged, and the sheet for electrophoretic display can easily be attached to another member.